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Materials 2019, 12, 1281 4 of 14 2.5. Stabilisation and Carbonisation of the Fibres The electro-spun fibre mats with dimensions of 12 × 5 cm2 were stabilised in an air-convection oven in four steps, each step taking 20 min in the range of 232–258 ◦C. Then, stabilised fibre mats were carbonised by heating in a tube furnace with a heating rate of 10 ◦C/min to reach 850 and 1200 ◦C, holding for 1 min. Both stabilisation and carbonisation processes were performed under tension by fixing the mats within a metal clamp and applying a ~30 g load. 2.6. Characterisations A rheometer (Discovery HR-3, TA Instruments, New Castle, DE, US). was used to measure the viscosity of the solutions as a function of shear rate ranging from 0.1 to 100 s−1 (40 mm diameter geometry, 2◦ cone angle in 49 μm truncation gap) at room temperature. Scanning electron microscopy (SEM) images were taken from the surface of the fibres using Zeiss Supra (Mönchhofallee, Kelsterbach, Germany).55VP at a 5 kV electron voltage. The fibre diameter of electro-spun fibres was measured using SEM images through image processing with Image J software and 100 measurements were conducted. Thermogravimetric analysis (TGA) measurements were obtained by a Perkin–Elmer TGA instrument (Waltham, MA, USA). at the heating rate of 10 ◦C/min under nitrogen atmosphere with a flow of 60 mL/min. Differential scanning calorimetry (DSC) analyses were conducted by a TA Q200 DSC instrument C at various heating rates under an air atmosphere. Brunauer, Emmett and Teller (BET) surface area was measured by a Micromeritics TriStar 3000 (Norcross, GA, USA) sing adsorption isotherm of nitrogen at 77 K. The X-ray diffraction (XRD) measurements were performed by a PANalytical X'Pert Pro (Jarman Way, Royston, UK) Diffractometer (Cu Kα radiation with λ = 1.54184 Å), operating at 40 kV and 30 mA with a step size of 0.033. Size of PAN and graphite crystallites in the pristine and carbonised electro-spun mat was calculated using the Scherrer equation according to the following: τ= kλ , (2) βcosθ where “k” is the shape factor of crystallites (0.9), "β" is full width at half maximum (FWHM) of the crystalline peak at a diffraction angle of "θ” and “τ" is the mean crystallite size. To calculate FWHM, a multiple peak fitting approach will be applied on XRD curves using Origin software to eliminate the effect of background inclination on the accuracy of results. The CF samples were analysed with Renishaw inVia Raman microscope and formation of disordered sp3 and graphitic sp2 carbon atoms (D- and G- bands, respectively) was investigated. Four random spots on each sample were picked to measure ID/IG ratios of samples. A conductivity meter (S30 Mettler Toledo SevenEasy) was used to measure the conductivity of the solutions. The electrical sheet resistance of the fibre mats was measured in fibre direction in 10 spots using a SZT-2A four-point probe tester. The molecular weight distribution of the CTP was determined using a Bruker (Billerica, MA, USA) Daltonics UltrafleXtreme MALDI TOF/TOF. The LDI-TOF-MS mass profile of the extract was then acquired using a smartbeam II laser (Nd:YAG, 355 nm) in positive, reflectron mode. 3. Results 3.1. Spinnability When it comes to processing and property relationships in electrospinning, different processing and materials factors including solution concentration, spinning atmosphere effects, accelerating voltage, distance from tip to target and solution viscosity play pivotal roles in determining the final properties of electrospun fibres. The viscosity of solutions can affect initiating droplet shape, jet trajectory, and fibre diameter. Moreover, the increase in fibre diameter can arise from the increment of the viscosity [29,30]. Based on this discussion, the effect of viscosity on the morphological aspects of spun fibres was studied. With respect to viscosity, the change of shear stress and viscosity according to shear rate are shown in Figure 2a,b. Moreover, the morphology of the electrospun fibre mats wasPDF Image | Low-Cost Carbon Fibre Derived from Sustainable Coal Tar Pitch and Polyacrylonitrile
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